1. Field of the Invention
This invention pertains to conversion of infrared radiation to electrical energy. More particularly, an indium-gallium-arsenide p-i-n photovoltaic cell is modified by insertion of strained quantum wells while avoiding lattice defects to increase conversion efficiency.
2. Description of Related Art
Thermophotovoltaic (TPV) energy conversion is a potentially environmentally friendly approach to achieve high efficiency, compact and reliable sources of electrical energy. In TPV conversion, a source of energy such as concentrated sunlight, nuclear power, fossil fuel, or a radioisotope heat source is used to heat an intermediate thermal emitter. The emitter then radiates photons which impinge on a photovoltaic cell.
There is now renewed interest in TPV energy conversion using nuclear energy sources or combustion driven systems operating at low temperatures (&lt;1500K). Possible uses include power generation for deep space exploration, silent portable gas or natural gas operated generators, non-polluting and silent energy generation for natural gas operated vehicles, power cogeneration (using conversion of waste heat into electricity), and peak electricity for power utilities or household use.
Common to all TPV systems operating at moderate emitter temperatures is the desire for low-bandgap photovoltaic devices that can convert efficiently the infrared-rich spectrum emanating from the emitter. In conventional photovoltaic cells the electron and hole result from absorption of a photon with energy above the bandgap. These carriers rapidly thermalize to their respective band edges. The fundamental efficiency limitation in a conventional cell results from the trade-off between a low bandgap, which maximizes light absorption and hence the output current, and a high bandgap, which maximizes output voltage. The spectral energy peak of a 1200 C (1500K) black body falls at a wavelength of 2 microns. As a result, for silicon photovoltaic devices only a very small portion (&lt;2%) of the emitted energy is above the bandgap and is available for PV conversion. Therefore the use of narrower bandgap semiconductors has been identified as a necessary condition to achieve higher efficiencies. Most of the existing development work is concentrated around two semiconductor systems: the ternary InGaAs cells fabricated on InP substrates and GaSb cells and the GaInAsSb quaternary alloys fabricated on GaSb substrates. However, currently, GaSb based technologies are only available in 2-inch diameter wafers, while the 3-inch diameter wafers used for InGaAs cells have twice the area, resulting in twice as many cells per processed wafer. High quality InP wafers are available from several competitive vendors due to the telecommunication industry's need for 1.5 microns detectors and lasers, while GaSb wafer production, limited to a much smaller TPV market, is still in a development stage. Consequently, U.S. companies and major government laboratories involved in TPV have mainly concentrated their research effort on the development of low-bandgap (0.55 to 75 eV) InGaAs devices fabricated on InP substrates. The initial effort was directed toward the fabrication of TPV cells using Ga.sub.0.47 In.sub.0.52 As with an energy bandgap of 0.75 eV and a material lattice constant matched to InP. These cells exhibit excellent PV characteristics; however, their efficiency for a 1500K spectrum is limited by transparency losses. The most recent research approach has promoted the use of narrower bandgap InGaAs (0.6-0.55 eV). In fact, for a 0.55 eV cell 35% of the black body energy is from photons with energy above the bandgap instead 14% for the Ga.sub.0.47 In.sub.0.52 As (0.75 eV) cell. However, these narrower bandgap cells are fabricated with materials presenting 1-2% lattice mismatch with respect to the InP substrates. The large lattice mismatch between the substrate and the device material leads to the generation of dislocations for thickness exceeding a few hundreds of Angstroms. A conventional P/N junction cell requires an active area thickness larger than 2 microns. The presence of dislocations results in a reduction of the minority carrier lifetime and hence leads to a poor performance. In order, to partially reduce the defect density in the device active region, 3-4 micrometer step-graded buffer layers or superlattices (U.S. Pat. No. 4,688,068) can be deposited prior to the active device growth. Incorporating these additional steps increases substantially the epitaxial process cost. Furthermore, the remaining dislocation density (&gt;10.sup.8 cm.sup.-2) may still lead to the aging of the TPV cell.
Recently, in the context of solar cells, it has been proposed that the use of periodic layers (quantum wells) in the active region of the device can enhance conversion efficiency. (U.S. Pat. No.5,496,415) What is needed is a process which both increases IR conversion efficiency of a conventional InGaAs-lattice-matched-to-InP thermophotovoltaic cell and prevents the generation of dislocations in the device.